The present invention relates to an electric power converting apparatus having a power converter connected to an AC power system and loads by way of a transformer, and more particularly to a technique for preventing a DC magnetization in the transformer.
FIG. 9 schematically shows an arrangement of a conventional power converting apparatus having a power converter connected to an AC power system via a transformer. A circuit for preventing a DC magnetization in the transformer is incorporated into the power converting apparatus. The power converting apparatus is disclosed in Japanese Patent Laid-Open Publication No. Hei. 7-28534.
In FIG. 9: reference numeral 1 is an AC power system as an AC power line; 2 is a self-excited converter for generating an AC voltage in response to a gate drive signal; 3 is a transformer inserted between the AC power system 1 and the self-excited converter 2; 4 is a DC voltage source for supplying a DC voltage to the self-excited converter 2; 5A and 5B are current detectors for detecting currents flowing through the windings of the transformer 3; 6 is a subtractor for computing a difference between the currents output from the current detectors 5A and 5B; 7 is a DC component detector for detecting a DC component of an output signal of the subtractor 6; 8 is a potential transformer which measures the voltage of the AC power system 1; 9 is a voltage reference circuit for producing a set voltage of the AC power system 1; 10 is a voltage command value generating circuit for generating a voltage command value to the self-excited converter 2 in accordance with the output signals of the voltage reference circuit 9 and the potential transformer 8; 11 is an adder for adding together an output signal of the DC component detector 7 and an output signal of the voltage command value generating circuit 10; 12 is a PWM (pulse width modulation) control circuit which determines an ignition timing of a self-extinction element in the self-excited converter 2 in accordance with the output signal of the adder 11, and generates a gate pulse on the basis of the determined timing; and 13 is a gate pulse amplifying circuit which amplifies an output signal of the PWM control circuit 12 and applies a gate drive signal to the self-excited converter 2.
An operation of the conventional power converting apparatus shown in FIG. 9 will be described.
In the power converting apparatus of FIG. 9, when a DC component is contained in the voltage of the AC power system 1 or the-output voltage of the self-excited converter 2, an exciting current containing the DC component flows into the transformer 3. The DC component contained in the exciting current magnetizes the transformer 3 to saturate the iron core of the transformer 3.
Of the winding currents of the transformer 3, the current flowing through the winding connected to the AC power system 1 is called a primary winding current, and the current flowing through the winding connected to the self-excited converter 2 is called a secondary winding current. An exciting current of the transformer 3 can be obtained by computing a difference between the primary winding current of the transformer 3 detected by the current detector 5A and the secondary winding current detected by the current detector 5B by the subtractor 6. The DC component contained in the exciting current, which will magnetize the iron core of the transformer 3, is obtained from the DC component detector 7 which is for detecting a DC component of the output signal of the subtractor 6.
The DC component of the exciting current, thus detected, is applied to the adder 11. The adder adds together the DC component and a voltage command value that is generated by the voltage command value generating circuit 10 in accordance with the output signals of the potential transformer 8 and the voltage reference circuit 9, and applied to the self-excited converter 2. The resultant signal output from the adder is used as a signal representative of a voltage-command-value correction value.
The PWM control circuit 12 forms a gate pulse signal in accordance with the output signal of the adder 11, and the gate pulse amplifying circuit 13 processes the gate pulse signal from the adder to form a gate drive signal. The gate drive signal is applied to the self-excited converter 2. In response to the gate drive signal, the self-excited converter 2 switches self-extinction elements contained therein in accordance with the output voltage of the DC voltage source 4, and produces a voltage corresponding to the output signal of the adder 11.
As described above, in the prior art power converting apparatus shown in and described referring to FIG. 9, the self-excited converter 2 produces a voltage corresponding to the output signal of the adder 11. Therefore, when a DC component is contained in the voltage of the AC power system 1 or the output voltage of the self-excited converter 2, the power converting apparatus operates in the following manner. That is, a DC component contained in the exciting current of the transformer 3 is detected, and applied to the adder 11. The self-excited converter 2 produces a voltage, which cancels the DC component contained in the voltage of the AC power system 1 or the output voltage of the self-excited converter 2, whereby to eliminate the DC magnetization of the transformer 3.
The prior art power converting apparatus constructed as mentioned above detects an exciting current of the transformer 3, and causes the self-excited converter 2 to produce a voltage corresponding to the detected exciting current. Therefore, a DC component contained in the detected exciting current of the transformer 3 is proportional to an output signal of the self-excited converter 2 which is representative of a voltage-command-value correction value for suppressing the DC magnetization.
A nonlinear correlation is generally present between an exciting current of the transformer 3 and a flux density of the iron core of the transformer 3, as shown in FIG. 10. A linear relation, expressed by a first order integration as given by an equation (1), is present between a voltage applied to the transformer 3 and a flux density of the iron core of the transformer 3. Therefore, a voltage to be output by the self-excited converter 2 when it receives the DC component of the detected exciting current must correspond to the exciting current/flux density relationship shown in FIG. 10. ##EQU1##
The conventional power converting apparatus does not include means for compensating for the nonlinear relationship between the exciting current and the magnetic flux of the transformer 3. Therefore, it is impossible to coincide the voltage of the self-excited converter 2 with the voltage necessary for suppressing the DC magnetization over the entire range of the flux density. Particularly in a flux density region where the DC magnetization in the transformer is large and at a point near to its saturation point, the difference between those voltages is great. Under this condition, it is difficult to sufficiently suppress the DC magnetization in the transformer.
The publication referred to above describes one of the solutions to the above problem. In the solution, a magnetic flux of the iron core of the transformer is directly detected by use of a Hall element. To this end, it is necessary to specially design and manufacture a transformer with the Hall element incorporated thereinto. To incorporate the Hall element into the transformer already assembled into the apparatus, it is necessary to alter the transformer. Sometimes, it is impossible to practically incorporate the Hall element into the transformer. If possible, its incorporation needs high cost and much time. As the size of the transformer becomes large, it is more difficult to incorporate the Hall element into the apparatus.